It is well known that electric fields distort around sharp points causing a concentration of field strength about such points. As a result, sharp extremities on tall buildings and air borne vehicles are particularly prone to being struck by lightning in a storm. This is a particular problem for aircraft which, in order to be more aerodynamic, often incorporate sharply radiused projections. One example of such a sharply radiused projection is the radome which is generally fitted to the nose of an aircraft.
The radome generally carries the radar system and other electro-magnetically sensitive equipment and is by necessity made from a dielectric material, consequently a lightning strike to the radome can result in disintegration of the radome and subsequent in loss of the aircraft through aerodynamic instability. Thus, aircraft are provided with lightning protection systems to limit the damage which may be caused in the event of a lightning strike to the radome.
Conventional protection systems are known as lightning diverters. These generally consist of metal strips extending from the tip of the radome, across its external surface and back towards the metal airframe of the aircraft. When lightning strikes the radome, the current is carried by the conducting strips to the metal airframe where higher current densities can be safely dissipated. More recent variations comprise what is known as a button strip. This consists of a row of closely spaced metal dots carried on a strip of dielectric material. Just prior to a lightning strike, the atmospheric electric charge surrounding the aircraft builds, the dielectric begins to ionise, thus initiating the electrostatic ionisation of surrounding air molecules. The metal dots increase their local field strength and form a plasma thus providing a conductive channel for conducting the current induced by the lightning.
During lightning strike the small quantities of metals used in these conductors are subject to extreme temperatures and electro-dynamic forces which tend to cause them to ablate. As a consequence, these systems have a “one strike” capability and must be replaced on landing.